As your campaigns are based on the amount of email contacts you can send to each month you can change who the recipients are at the beginning of each month.
How do you know which contacts are included?
1. Create a group with the contacts you wish to send to for the month
2. At the top of the group details you can view how many contacts are within that group
Note: If you try sending a campaign to more contacts that your monthly limit allows you will receive an error message. You can increase the send limit if necessary by visiting the billing portion of the account.